1. Field of the Invention
This invention relates to a process for the treatment of waste water containing boron compounds and radionuclides.
Waste water of this type occurs, for example, as evaporator concentrate in nuclear power plants equipped with a pressurized water reactor.
For the disposal of radioactive material, it is necessary to achieve the greatest possible reduction of volume, since the storage space available is extremely limited. This is particularly true for radioactive waste water.
2. Description of the Prior Art
German Laid Open Patent Application No. 17 67 999 discloses a process for evaporating radioactive waste water, or chemically precipitating the radionuclides contained in the waste water.
For waste water which has a high concentration of boric acid, however, the evaporation does not achieve the desired success, since the remaining substances still contain large quantities of non-radioactive salts which are difficult to precipitate out. Major problems are also presented by complex-forming radioactive antimony (Sb-124) which has a half-life of approximately sixty days.
German Patent No. C-2723025 and "ABC-Chemie", Volume 1, 2nd Edition (Verlag Harry Deutsch, Frankfurt, 1970, page 198) each disclose a process to react boric acid and methanol with concentrated sulfuric acid as a catalyst to form boric acid trimethyl ester and water. After a separation, water is used to hydrolytically split the boric acid trimethyl ester into boric acid and methanol. This process was used to treat radioactive waste water from nuclear power plants containing boric acid and radioactive antimony. This process is unsatisfactory in several respects: Methanol, with air and ester, forms an explosive mixture which is flammable within broad limits. Methanol itself is highly volatile and toxic, and can therefore be permitted in the workplace in only very low concentrations. Furthermore, methanol and boric acid ester form an azeotropic mixture which is very complex, time-consuming and expensive to thermally separate. The sulfuric acid also represents a problem. Since all the radionuclides and the non-radioactive trace elements collect in the sulfuric acid, these substances must be removed from the sulfuric acid. Sulfuric/acid is also corrosive and expensive.
German Patent No. A-2252717 likewise shows the application of methanol for the esterification of boric acid in waste water.
European Patent No. A-0125017 discloses the usage of the alcohols methanol, propanol, isopropanol and mixtures thereof for reaction with boric acid to form boric acid esters. These alcohols also form an azeotrope with the boric acid ester, and these azeotropic mixtures are also difficult, time consuming and expensive to separate.